1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly to an optical pickup device suitable for a compatible optical pickup device for emitting several laser light beams having different wavelengths to a recording medium.
2. Description of the Related Art
Currently, various optical discs such as a compact disc (CD) and a digital versatile disc (DVD) have been commercialized and widely used. Further, recently, standardization of a next-generation DVD for recording and reproducing information using a blue-violet laser light beam has been proceeded. In the next-generation DVD, information is recorded and reproduced using the blue-violet laser light beam having a wavelength of about 405 nm. When the wavelength of the laser light beam shortens, a higher density can be obtained.
Therefore, when the variety of optical discs increases, development of a so-called compatible optical pickup device capable of performing recording and reproduction on different kinds of optical discs is demanded. In this case, an arrangement in which a plurality of laser elements having different emitting wavelengths are provided all together in a single CAN package can be achieved. According to such arrangement, a space for disposing semiconductor lasers can be reduced and an optical system can be commonly used among the laser light beams.
However, when the plurality of laser elements are provided in the single CAN package as described above, a deviation occurs between the optical axes of the laser light beams according to arrangement gap between the respective laser elements. Thus, when the optical axis of the optical system is aligned with the optical axis of a laser light beam, the optical axes of other laser light beams deviate from the optical axis of the optical system. Consequently, in the case of recording and reproduction using the other laser light beams, there arises a problem in that aberration of laser light beams is produced on a recording medium to cause deterioration of optical characteristics.
Therefore, according to the related invention as disclosed in JP 06-131688 A, a birefringence element is disposed immediately after a semiconductor laser including several kinds of laser elements, and the optical axes of the laser light beams are aligned with one another by the birefringence element to guide the laser light beams to the optical system.
The related invention requires an additional birefringence element. In addition, it is necessary to form the respective laser elements such that the polarization plane of a reference laser light beam is orthogonal to the polarization plane of another other laser light beam. However, it is not easy to form the laser elements in which the polarization planes of the laser light beams are different from each other. Because the birefringence element is expensive, a problem in that a cost of the entire optical pickup device increases occurs.
Therefore, Japanese Patent Application No. 2004-145169 has been filed by the applicant of the present invention to propose an optical axis correcting technique using a diffraction grating. According to this related invention, the diffraction grating is used as an optical axis correcting element, so an increase in cost can be suppressed. In addition, it is unnecessary to adjust the process to form laser elements while considering the state of the polarization plane of each laser beams having different kinds of wavelength, unlike JP 06-131688 A.
As described above, when the three laser elements for emitting a laser light beam for CD, a laser light beam for DVD, and a laser light beam for next-generation DVD are housed in the same CAN package, the optical axis of the laser light beam for next-generation DVD is normally aligned with the optical axis of the optical system in priority to the other laser light beams. This is because the wavelength of the laser light beam for next-generation DVD is short and the emitting power thereof is weak.
In contrast to this, there is a case where an objective lens is designed so as to become a finite system for only the laser light beam for CD, of the three laser light beams. In such a case, when the optical axis of the laser light beam for CD is deviated from the optical axis of the objective lens, optical properties of the laser light beam for CD significantly deteriorates as compared with a case where the other laser light beam is deviated therefrom.
In order to suppress the deterioration of optical properties by the off-axis, a method of tilting the objective lens has been known. However, when the laser light beam is incident on the objective lens as a finite system light beam as in the case of the laser light beam for CD, the deterioration of the optical properties due to the deviation of the optical axis becomes significant. According to the study of the inventor of the present invention, when the laser light beam is incident on the objective lens as the finite system light beam as in the case of the laser light beam for CD, it is determined that the deterioration of the optical properties cannot be sufficiently compensated even if the tilting is performed. In contrast to this, when the laser light beam is incident on the objective lens as an infinite system light beam, the deterioration of the optical properties can be compensated by the tilting.